Bluetooth SIG officially released a new core supplementary specification (Core Spec 2.1 CSA1) containing eL2CAP (ERTM/SM mode) on Jun. 26, 2008. The most important change of the next generation Bluetooth core specification (Seattle version) is the introduction of AMP (alternative physical layer) architecture, which is an innovative wireless technology (RF) substitution rule. The new specification allows the Bluetooth protocol stack to choose proper wireless technology dynamically according to different tasks. In addition to the conventional Bluetooth 2.1+EDR, the new specification also provides support for high-speed transport layer such as 802.11 (WiFi). Bluetooth high-level protocols and applications are not required to make changes, thus ensuring a smooth upgrade of existing products during the update of Bluetooth core specification.
In the time of conventional Bluetooth controller, the Bluetooth controller is responsible for error correction and retransmission, and reliability of data transmission is ensured without the involvement of the host. Under the Bluetooth AMP architecture, the controller of high-speed transmission media is no longer responsible for the reliability of data transmission, and the host is required to provide the instant error correction and retransmission mechanism. However, traditional RT (retransmission)/FC (flow control) mode of the Bluetooth L2CAP has a design flaw; when a loss of packet is detected by the sender, all unresponsive frames have to be retransmitted, using no more than half of the transmission window. The core supplementary specification has an additional ERTM (Enhanced retransmission)/SM (stream) mode and has upgraded the original RT (retransmission)/FC (flow control) mode to fix the design flaw. The changes mainly focus on: providing additional SREJ (selective reject frame)/RNR (receiver not ready) control frame and providing additional Poll-Final bit field; when a loss of frame is detected, the sender first send inquiries about the current receiving status of the receiver by a RR (Poll=1) message and then decides the retransmission strategy to be used, so that extra transmission due to forced consecutive retransmission on detection of a loss of frame under RT model may be avoided, with full-window transmission and greatly increased speed compared to the maximum-half-window transmission in the RT model. The ERTM is still a slide window transmission model based on the transmission-response mechanism.
During the data transmission between the sender and the receiver, the data includes information frame (I-frame) and supervisory frame (S-frame). I-frame is used to transmit user data, and meanwhile carrying flow control and error control information to guarantee correct data transmission. S-frame is dedicated to transmit control information; when the flow control and error control information can not be carried by I-frame, S-frame is used. ACK (acknowledge) is a transmission control character sent from the receiver to the sender during the data transmission, which is used to acknowledge the correct reception of I-frame: on receipt of an ACK signal, the sender could then send the next group of frames; if the sender fails to receive ACK signal before RTX timeout occurs, it will retransmit the current data package or stop the data transmission. ACK can be carried by either I-frame or S-frame.
For the timing to send ACK response frame, there is no strict rules defined in specification; implementer could decide how to send ACKs. A conventional practice is to send ACKs when receiving an I-frame, whatever local have I-frame to send or not, which will decrease the transmission speed due to redundant transmission caused by too frequent ACKs. In contrast, if the ACK response is not established actively, the sender will wait for an RTX retransmission timeout to send Poll frame to inquire, which will cause passive ACK response from the receiver; in this case, the sender can not send any data before an ACK is received to clear unresponsive send window (send outstanding window), thus slowing down the transmission speed. Therefore, it has become a key point in realizing ERTM mode as to design an optimal ACK transmission mechanism, which can improve the data transmission speed.